The crew's four astrophysicists spent most of the day readying the telescopes for astronomical observations, expected to begin today.

During the 15 1/2-day flight--the longest in shuttle history--the telescopes will study ultraviolet light streaming from stars, galaxies, quasars, the moon and Jupiter. Astronomers want to understand how the universe began and how it might end.

Their No. 1 quarry is elusive, if it exists at all. For years, astronomers have been trying to find gaseous helium believed to have been formed when the universe was created.

The astronauts will aim the telescopes at a quasar 10 billion light-years away to see if ultraviolet light radiating from the quasar is absorbed by anything. If it is, the absorbent material could be intergalactic helium.

"If we just succeed in getting good enough observations of that one object, I'll be satisfied with the mission," said Arthur Davidsen, an astrophysicist at Johns Hopkins University who is in charge of one of the telescopes.

Astronomers have more than 600 celestial targets from which to choose. Endeavour's astrophysicists will work around the clock in 12-hour shifts to squeeze in as many observations as possible before the flight ends on March 17.

Earth's atmosphere blocks out most ultraviolet radiation, so scientists have to study it from space.

The observations by the $200-million set of telescopes known as Astro will complement--not duplicate--the work of the $1.6-billion Hubble Space Telescope.

The astronauts had to shut the telescope doors for an hour Thursday afternoon because of a small jet leak. The jet was turned off, eliminating any contamination to sensitive telescope parts.

It is the second consecutive shuttle flight to be bothered by a leaking thruster.

Endeavour and its crew of seven lifted off at 1:38 a.m., taking advantage of a clearing in the sky.

Shuttle commander Stephen Oswald was taking no chances. Two minutes before liftoff, he asked that NASA's deputy director of flight crew operations, Steven Hawley, disguise himself by putting a bag over his head. In his astronaut days, Hawley set the record for most shuttle launch delays--10.